This invention relates generally to ignition or spark timing control systems for use in internal combustion engines, and more particularly to a method and apparatus for controlling spark timing for each cylinder of engine so that the ignition is effected with high precision.
Various types of electronic ignition timing control system have been developed with a view to maximizing the thermal efficiency of internal combustion engines to meet increasing demands for energy savings.
Conventional ignition timing control systems are arranged such that the ignition is performed after the elapse of a predetermined time in response to a reference position signal which is obtained in response to the piston position in each engine cylinder reaching top dead center (TDC). The reference position is established at a position advanced from the TDC.
In the conventional systems, there is a problem associated with the imprecision of spark timing in a transition period, particularly, in the case that the spark timing is established at time elapsed much from the reference position.
According to another prior art approach, as is described in Japanese Laid-open Application No. 57-195867, there is a control system of spark timing comprising: means for generating a reference pulse having a leading edge advanced as a function of the rotational speed of engine; and control means for computing, using the engine load and engine speed, a spark timing as a period of time elapsed from the leading edge of the reference pulse and for effecting the ignition in accordance with the computation.
On the plus side, the prior art system of second type is being appraised successfully with regards to the prevention of the decrease of engine output when the engine is accelerated and to the prevention of the aggravation of acceleration feeling. On the other hand, as a problem associated with the second prior art, the position of leading edge of reference pulse signal is varied because of the difference in performance of sensor for detecting the reference position such as a predetermined engine crankshaft position, cyclic variaion of each cylinder, eccentricity of drive system, and the like, resulting in a poor spark timing precision. For a better understanding of the problem associated with the prior art, are illustrated FIGS. 1A and 1B showing reference pulses generated by a waveform shaping circuit and pulse signals causing the ignition to occur. It is seen from FIG. 1A that the position of leading edge of pulse is varied, and therefore the pulse duration are also varied with cycle because the position of trailing edge of the reference pulse is stabilized. As a result, the spark timing is similarly varied as shown in FIG. 1B. The reason is that the spark timing is determined in accordance with the pulse duration. In FIG. 1B, the numerals represent cylinders to be ignited in response to the pulse signal.